1. Field of the Invention
The present invention relates to optical glass which has a high refractive index, a high dispersion and further a low glass transition temperature (Tg), and is suitable for the precision press molding (the precision mold press forming).
2. Related Art
In recent years, with the advent of digital cameras, the degree of integration and the high degree of functionality of devices employing optical systems advance rapidly. At the same time, the demand for a high precision, lightweight, compact optical system has been increasing. To meet the above demand, optical designs employing aspherical lenses have been increasingly coming in the mainstream. Then, in order to stably supply a large amount of aspherical lenses employing a highly functional glass at a low cost, a mold pressing technique in which an optical surface is directly formed without steps of grinding or polishing has attracted attention. The demand for optical glass having high functionality (for example, a high refractive index and low dispersion properties/high refractive index and high dispersion properties and the like) has been increasing year by year.
The precision press molding of glass is a method to obtain a glass form having a shape of the final product or extremely close to the final product, and with face precision, and according to the precision press molding, a mold with a desired shape can be manufactured with a high rate of productivity. Then, at present, many kinds of optical glass parts such as spherical lenses, aspherical lenses, diffraction gratings, and the like are manufactured by precision press molding. Not surprisingly, in order to obtain an optical glass part by precision press molding, it is necessary to perform press forming of a glass preform under high temperature. Thus, the mold used for the press is exposed to a high temperature, and high pressure is applied thereto. For the above reason, from the view point of suppressing damage to the mold itself or the parting film provided on the inside surface of the mold, caused by the high temperature environment of the press molding, the glass transition temperature of the glass preform is preferred to be as low as possible. In addition, devitrification is not eliminated in the glass preform material by precision press molding of a glass preform material in which devitrification has formed, and a glass mold containing devitrification cannot be used as an optical element. Thus, a high resistance to devitrification is required for the glass of a glass preform material to be used in the precision press molding.
Conventionally, an optical glass having a high refractive index and high dispersion is represented by a composition containing a large amount of lead oxides, and has superior stability and a low glass transition temperature, and thus has been used for precision mold press forming. For example, the Japanese Patent Application, First Publication No. 1989-308843 discloses an optical glass containing a large amount of lead oxides used for the precision mold press forming.
However, in performing the precision mold press forming, it is maintained in a reductive atmosphere in order to protect the mold from oxidation, and therefore, when lead oxide is contained in the glass component, there is a problem in that the precision face of the die could not be maintained due to the adhesion of the reduced lead precipitated on the surface of the glass to the surface of the die. In addition, since lead oxide is harmful to the environment, glass that does not contain lead oxide has been desired.
Then, as the optical glass for the press molding that has a high refractive index and a high dispersion and does not contain lead oxide, various kinds of glass containing TeO2 component and the like have been developed, such as those disclosed in Japanese Patent Application, First Publication Nos. 2004-43294, 2005-154251 and 2006-182577.